The technique of building up a "primary" preform by plasma deposition is relatively recent. It is used in particular for making monomode optical fibers. It makes it possible to obtain a preform of increased diameter after it has been built up, thereby making it possible to draw considerably longer lengths of optical fiber from the built-up preform.
The most commonly used prior technique for building up a primary preform is known as "sleeving". It consists in placing the primary preform in a tube selected to be of a kind that is as similar as possible to the outer layer of the primary preform, and in collapsing the sleeving tube onto the primary preform. Once the silica sleeving tube has been collapsed, that method makes it possible to obtain a final preform that has a diameter of about 40 mm, and it also makes it possible to obtain optical fiber, and in particular monomode fiber, by drawing fiber from the preform built up in that way, which fiber has acceptable attenuation at wavelengths that are useful for transmission purposes. However, that method has a main drawback of requiring a final operation of collapsing the sleeving tube onto the primary preform. It is also limited by the small range of sleeving tubes that are commercially available, which tubes are made essentially either of "pure" silica or else of silica that has been doped with fluorine. In order to ensure compatibility between the outer layer of the primary preform and the sleeving tube, that technique is usually applied to modified chemical vapor deposition (MCVD) primary preforms in which the optical cladding and the optical core are made in a commercially available tube of "pure" silica, and then using a sleeving tube which is likewise made of commercial "pure" silica.
In comparison therewith, the technique of building up a preform with pure silica or with doped silica by plasma deposition has the advantage of causing the building-up silica to vitrify directly on the primary preform. It also has the advantage of being capable of being implemented on a preform made by any of the various known methods, such as vapor axial deposition (VAD), outside vapor deposition (OVD), or modified chemical vapor deposition (MCVD), in particular. It thus makes it possible, as does the sleeving technique, to increase the thickness of the outer layer of the primary preform, with the material built up by plasma deposition then being as similar as possible to that of the outer layer of the primary preform. Another advantage over the sleeving technique is that it enables all of the outer covering of the final preform to be built up on a primary preform that initially comprises only the optical core and the optical cladding, or on a primary preform, in particular an MCVD preform, that initially comprised a tube in which the optical cladding and the core were made by internal deposition, after which the tube was collapsed and the material from the original tube was removed before the building-up operation. When all of the outer covering is made in this way, the build-up material is selected to have a refractive index that is suitably different from that of the optical cladding which it covers.
Until now, that technique of external plasma deposition of silica has been implemented while reducing as much as possible the presence of hydroxyl ions in the deposited silica.
Thus, document FR-A-2 253 723 describes a method of preparing a preform, particularly for implementing external deposition of silica on a tube or a rod, starting from a fine desiccated powder of pure silica, which is fed to the flame of a plasma torch, and using a plasma-generating gas, preferably argon, while protecting the deposited silica from any kind of pollution (and particularly OH-ion pollution) by installing the preform with its silica deposit in an oven enclosure in which a high vacuum is established.
The Applicant has observed that building up a relatively large thickness by plasma deposition in that way gives rise to a final preform, and thus to fibers derived from said final preform, in which attenuation is high. Attenuation increases as a function of the final diameter of the build-up preform and it rapidly becomes excessive, thereby putting a major constraint on the use of that technique in the past.